This invention relates to communication systems and more particularly to radio communication systems which operate to conserve radio frequency spectrum by controlled sharing of one or more narrowband communication channels with a plurality of users and which are not dependent for such operation upon the use of a central control apparatus or the use of special signals for marking channel occupancy.
With an ever increasing need and use of commuication systems and particularly those systems using radio as the transmission link, the problem of providing necessary space in the radio frequency spectrum for each new user is fast becoming insolvable.
In the past, numerous methods of radio communication have been devised to conserve radio frequency spectrum. These prior art schemes have included time and frequency division systems and hybrid combinations of these systems as well as complex signal correlation methods to achieve access and address capabilities. These prior art systems, for the most part, have been complex in nature and costly and generally have not been entirely satisfactory or practical to implement as evidenced, in part, by the general lack of acceptance and utilization in the communication field.
Another solution to the spectrum problem has been described in "Communications System Engineering Handbook" by Hamsher and published by McGraw-Hill, reference page 18-5. In this solution, as in the present invention, each user is provided with access to a common pool of communication channels. Further, in this solution, the user selects and occupies a free channel and protects its' use from others by the use of a "busy" signal. The user can then ring a desired user and can communicate with him on the acquired channel.
The use of so called "busy" or "off hook" signals to mark an occupied channel as described by Hamsher is not desirable. Continuous transmission of such signals require transmitters capable of continuous duty which from an economic standpoint, greatly increases the cost of the transmitting equipment. The need of a tone control marker also implies a costly central control station. The use of such signals to mark an occupied channel, whether or not the signals are continuous or intermittent in nature also increases the complexity of the system since a means must be provided to both transmit and receive the signal along with the normal communication signals. The time duration of the transmitted signal is also important since the shorter the time duration, the greater the probability of misdetection. Such misdetection could occur for example, when short bursts of signal are transmitted at the beginning and/or end of each communication signal transmission. In view of this possibility, the detectability criterion of the busy signals should be well below the threshold of the normal voice or communication signals. This requirement, of course, increases both the complexity and cost of the system. The transmission of such "busy" signals for any time during other than continuous also greatly increases the probability of not receiving the signal at all, during a sampling period when the communication channels are periodically sampled for occupancy status as in the herein disclosed present invention.
In view of the foregoing, it should now be understood that it would be desirable to provide an improved communication system that would solve the above and other problems.
Accordingly, one of the objects of the present invention is to provide a communication system which enhances the use and efficiency of one or more communication channels which is simple and reliable in operation and which is particularly suitable for use with existing narrow band communication channel assignments.
It is another object of the present invention to provide a communication system in which sharing of one or more communication channels by a plurality of users is controlled solely by user apparatus, thereby not requiring a central control station.
It is yet another object of the present invention to provide user apparatus for sampling the communication signals on each one of a number of communication channels and for controlling the transmission of signals from the user apparatus on each of the sampled channels as a function of the sampled communication signals.
It is still another object of the present invention to provide a user apparatus for periodically sampling the communication signals on one or more communication channels for detecting channel occupancy and prohibiting transmission of signals from the user apparatus on each one of the channels detected as occupied for a predetermined length of time after the detection of communication signals.
Studies of private radio communication networks such as for example those used in public safety, industrial, and land transportation radio services indicate that greater than 66 percent of the communication channels presently assigned by the FCC to such users are actually in use less than 10 percent of the total possible use time. Such limited and inefficient use of the communication spectrum is, of course, undesirable from the standpoint of spectrum conservation.
The present invention takes advantage of communication channels with these relatively low duty cycle users by sharing them with other low duty cycle users. The disclosed system operates with minimal operator control and is not dependent upon a central control equipment or the use of special signals for identifying a channel occupancy, thus there is provided a practical, simple, and relatively inexpensive solution for increasing the use efficiency of existing communication channels with low duty cycle users.
The sampled communication system of the present invention uses a channel-scanning approach to define the availability of a channel. Also, the channel is sampled and the information is held for decision purposes. The capability is achieved by local control at the transceiver thereby not requiring a central control station.